Modern short-reach optical communication systems, such as those used in data centers, require high data rates together with low cost and energy consumption. To improve spectral efficiency and approach channel capacity, probabilistic constellation shaping (PCS) has emerged as a promising technique by adapting the probability distribution of transmitted symbols.

While PCS is well established for coherent systems, its application to intensity modulation/direct detection (IM/DD) systems is more challenging due to constraints such as signal non-negativity and the integration with forward error correction (FEC). In [1], the authors propose a practical PCS scheme for IM/DD systems by modifying the bit-to-symbol mapping, enabling a near-optimal distribution and demonstrating measurable performance gains.

However, many PCS approaches are derived under average power constraints. In practical IM/DD systems, hardware limitations such as the finite extinction ratio impose a peak power constraint, which can significantly alter the optimal input distribution. This aspect is investigated in [2], where probabilistic shaping is analyzed under peak power limitations.

Furthermore, recent studies such as [3] show that classical probabilistic shaping assumptions may not directly translate to IM/DD systems, highlighting the importance of system-specific constraints in shaping design.

The goal of this seminar is to understand the implementation of PCS in IM/DD systems as presented in [1], and to analyze how peak power constraints and system limitations affect shaping performance, as discussed in [2] and [3].

[1] Zonglong He, Tianwai Bo, and Hoon Kim, "Probabilistically shaped coded modulation for IM/DD system," Opt. Express 27, 12126-12136 (2019)

[2] T. Wiegart, F. Da Ros, M. P. Yankov, F. Steiner, S. Gaiarin and R. D. Wesel, "Probabilistically Shaped 4-PAM for Short-Reach IM/DD Links With a Peak Power Constraint," in Journal of Lightwave Technology, vol. 39, no. 2, pp. 400-405, 15 Jan.15, 2021, doi:10.1109/JLT.2020.3029371

[3] D. Che, J. Cho and X. Chen, "Does Probabilistic Constellation Shaping Benefit IM-DD Systems Without Optical Amplifiers?," in Journal of Lightwave Technology, vol. 39, no. 15, pp. 4997-5007, Aug.1, 2021, doi: 10.1109/JLT.2021.3083530.

Prerequisites